Water in hydrocarbon formations may provide a growth media for anaerobic bacteria. Certain anaerobic bacteria, such as SRB, may be problematic in recovery of hydrocarbons from hydrocarbon-bearing formations. For instance, SRB may reduce sulfates to sulfides, which may damage the hydrocarbon-bearing formation. In addition, SRB may form slimes or sludges, reducing the porosity of the formation. Reducing the porosity of the formation may impede recovery of the hydrocarbons from the hydrocarbon-bearing formation. Reduction of porosity may be a particularly acute problem in low porosity formations, such as shale.
Fracturing operations may be used to increase hydrocarbon recovery from hydrocarbon-bearing formations. Fracturing operations make use of fracturing fluids, which are often water-based. Depending on the formation and the fracturing operation method, water-based fracturing fluid may be retained in the formation for extended periods. For instance, small-pore sized, low-porosity shales may retain a significant amount of water-based fracturing fluid. The water retained in the formation from the fracturing operation may provide a growth media for SRB.
Traditional water-based fracturing fluids may include a biocide to control SRB. However, biocides, in particular long-acting biocides such as glutaraldehyde, may present environmental concerns, such as ground water contamination. Short acting biocides, such as oxidizers, may present less of an environmental hazard, but may not be active over the entire time period in which the fracturing fluid is retained by the hydrocarbon-bearing formation.
Nitrate-Reducing Bacteria (NRB) may inhibit the growth of SRB by using a more efficient nitrate-reduction metabolic pathway than SRB and removing nutrients from the environment that SRB require in order to grow and produce sulfides. Traditionally, NRB require a high concentration of nitrate in water in the hydrocarbon to be effective and, thereby necessitating large quantities of nitrate to be pumped simultaneously with the NRB into the hydrocarbon-bearing formation.